This screengrab taken on May 12, 2014, from a video of Nigerian Islamist extremist group Boko Haram obtained by AFP shows girls, wearing the full-length hijab and praying in an undisclosed rural location.<br />Nigeria’s government on MAy 17, 2017 said another Chibok girl was free, having escaped from Boko Haram Islamists more than three years after being kidnapped with more than 200 classmates. / AFP PHOTO / BOKO HARAM / HO

A group of young women shown in a video released by Boko Haram on Monday are purported to be among the more than 200 abducted from Chibok, northeast Nigeria, on April 14, 2014.

Here is a timeline of the mass kidnapping that captured global attention:

– Snatched from school –

On April 14, 2014 Boko Haram gunmen seize 276 girls aged 12 to 17 from the Government Girls Secondary School in the remote town of Chibok in Borno state.

The girls are forced from their dormitories onto trucks and driven into the bush. Fifty-seven manage to flee.

Boko Haram factional leader Abubakar Shekau claims responsibility in a video released on May 5 and vows to sell the girls as slave brides.

One year on

On April 14, 2015 Nigeria’s president-elect Muhammadu Buhari warns he “cannot promise” that the girls will be found, as vigils are held to mark their first year in captivity.

In September 2015, Buhari raises the possibility of an exchange of Boko Haram prisoners for the girls.

In December he says he is willing to negotiate with any “credible” Boko Haram leadership and if there is proof the girls are alive.

‘Proof of life’

In April 2016, on the eve of the abduction’s second anniversary, it emerges that Boko Haram has sent a “proof of life” video to the government.

It shows 15 of the girls in black hijabs in the first concrete indication that at least some are still alive.

In May 2016 the Nigerian army confirms the first of the schoolgirls has been found.

The 19-year-old was discovered with a four-month-old baby and a man she described as her husband near Boko Haram’s Sambisa Forest enclave.

– First releases –

In October 2016, Nigerian officials announce the release of 21 of the girls following talks between the government and Boko Haram, brokered by Switzerland and the International Committee of the Red Cross.

Local sources say four jihadist prisoners were freed as part of the deal.

The army finds two other girls in November 2016 and January 2017.

In May 2017 another 82 girls are released in exchange for five Boko Haram commanders as part of the same talks.